Are you gettting telephone calls from 954-623-4620? So have LOTS of other people!
It just so happened that I saw mention of this on MetaFilter on the day that I have received at least 2 hang-up calls with nothing but a few weird tones when I pick up the phone.
It seems AT&T has hired a telemarketing company to get people to switch to AT&T. Unfortunately, they are using the most annoying methods possible and, in some cases, refusing to honor DO NOT CALL requests.